This invention relates to a method for reducing the ash-content of lubricating oil containing ash-forming components. In another aspect this invention relates to a method for the treatment of used lubricating oils to obtain purified oil suitable for use as fuel oil, in grease formulations, or in the preparation of lubricating oil formulations.
Used motor oil has been estimated as being generated in the United States at a rate of about 1.1 billion gallons per year. Some of this used oil has been used as furnace oil and some has been used on rural dirt roads for dust control. Much of the oil has been merely discarded in sewers, dumps, and back alleys. With the ever decreasing petroleum reserves, it becomes more and more essential that this used oil be saved and used as long as possible.
One major obstacle to re-use of used oil in many applications involves the presence of various ash-forming impurities that remain dispersed in the oil due to the very effective dispersant characteristics of the additives in modern day lubricant systems.
Materials contained in a typical used crankcase oil that are considered to contribute to the ash content of the oil include sub-micron size carbon particles, inorganic materials such as atmospheric dust, metal particles, lead and other metal compounds originating from fuel combustion. Besides lead, which is generally present at concentrations of 1.0 to 2.5 weight percent, appreciable amounts of zinc, barium, calcium, phosphorus and iron are also present in the used crankcase oil. Examination of the used oil under an optical microscope at 600 magnifications reveals the very effective dispersant characteristics of modern day lube oils. The particle size of the particulates is estimated from this microscopic examination to be 0.1-1.0 microns with virtually no occurrence of agglomerates in the oil.
The presence of the ash-forming components in used oil puts limits on the extent to which the material can be used economically without ecological damage. For example, reuse of the used oil as fuel oil can give rise to serious atmospheric pollution when the oil contains in excess of one percent lead. Also, such fuel oil often results in burner and refractory maintenance costs that offset the purchase price differential between used oil and regular furnace oil.
Clearly, it is in the national interest to provide economical ways of removing the impurities from used oil so that it can be reused practically.
Recently, a technique of purifying used oil has been developed in which the used oil is reacted with an aqueous solution of an ammonium salt treating agent, then the water phase is removed, and the resulting oil phase-containing mass is separated by filtration. Such a technique is described in U.S. Pat. No. 4,151,072, the disclosure of which is incorporated herein by reference.
An improvement upon the process of U.S. Pat. No. 4,151,072 is disclosed in U.S. Pat. No. 4,247,389, the disclosure of which is also incorporated herein by reference. The improvement of this latter patent involved the use of a heat soak step wherein the oil after having the water removed was allowed to soak for a time at a relatively high temperature prior to being filtered. The high temperature heat soak step resulted in an improvement in the amounts of ash-forming components that could be separated from the oil.
In the initial commercial development of the process of U.S. Pat. No. 4,247,389, the oil after having substantially all the water removed was heated to the soaking temperature by being passed through a heating coil located in a furnace and then passed to a heat soaking vessel wherein a large volume of the hot oil was maintained at an elevated temperature for the required length of time. One drawback in this process was the fact that even though the oil was not being heated to a temperature at which any significant amounts of cracking would be expected to occur, some fouling of the heating coil did occur possibly as a result of the formation of decomposition products which adhered to the walls of the heating coil. In some instances, the extent of the fouling was so serious that it became the limiting factor on the length of time that the process could be effectively carried out.
An object of the present invention is to provide an improved method for carrying out the heat soak step in such an oil refining process.